Soft hand-off in cellular mobile communications networks

ABSTRACT

In a cellular mobile communications network a mobile station is capable of receiving a downlink signal from each of a plurality of base stations and transmitting an uplink signal to the plurality of the base stations through a wireless channel. The mobile station produces a measure of signal quality of the downlink signals from the plurality of base stations to the mobile station and selects a base station from which the downlink signal shows a preferred signal quality. The mobile station transmits an uplink signal indicating the selected base station among the plurality of base stations for subsequent communications with the mobile station. Each base station processes the uplink signal to identify the selected base station from among the plurality of base stations.

This application is a divisional of Ser. No. 09/696,574, filed Oct. 25,2000, now pending, which is a continuation of PCT application Serial No.PCT/GB99/01347, filed on Apr. 28, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to cellular mobile communication networks,for example Code Division Multiple Access (CDMA) cellular networks.

2. Description of the Prior Art

FIG. 1 of the accompanying drawings shows parts of a cellular mobiletelecommunication network according to the Telecommunication IndustriesAssociation (TIA)/Electronic Industries Association (EIA) StandardTIA/EIA/IS-95 of October 1994 (hereinafter “IS95”). Each of three basetransceiver stations (BTSs) 4 (BTS1, BTS2 and BTS3) is connected via afixed network 5 to a base station controller (BSC) 6, which is in turnconnected to a mobile switching center (MSC) 7. The BSC 6 serves tomanage the radio resources of its connected BTSs 4, for example byperforming hand-off and allocating radio channels. The MSC 7 serves toprovide switching functions and coordinates location registration andcall delivery.

Each BTS 4 serves a cell 8. When a mobile station (MS) 10 is in aso-called “soft hand-off” (SHO) region 9 where two or more cellsoverlap, a mobile station can receive transmission signals (downlinksignals) of comparable strength and quality from the respective BTSs ofthe overlapping cells. Transmission signals (uplink signals) produced bythe mobile station (MS) can also be received at comparable strengths andqualities by these different BTSs when the mobile station is in the SHOregion 9.

FIG. 2 of the accompanying drawings shows a situation where the MS 10 islocated within the SHO region 9, and is transmitting such uplink signalsthat are being received by plural BTSs 4. According to the IS95standard, a BTS 4 that receives such an uplink signal from the MS 10relays the signal to the BSC 6 via a dedicated connection line of thefixed network 5. At the BSC 6, one of the relayed signals is selectedbased on a comparison of the quality of each of the received signals,and the selected signal is relayed to the MSC 7. This selection isreferred to as Selection Diversity.

Similarly, FIG. 3 of the accompanying drawings shows a situation wherethe MS 10 is located within the SHO region 9 and is receiving downlinksignals from plural BTSs 4. According to the IS95 standard, downlinksignals received by the BSC 6 from the MSC 7 are relayed to all BTSs 4involved in the soft hand-off via respective connection lines of thefixed network 5, and subsequently transmitted by all the BTSs 4 to theMS 10. At the MS 10 the multiple signals may be combined, for example,by using maximum ratio combination (MRC), or one of them may be selectedbased on the signal strength or quality, i.e. using Selection Diversityas for the uplink case.

In contrast to, for example, Global System for Mobile Communication(GSM) networks, in CDMA networks each BTS 4 transmits at the samefrequency. Consequently, careful control of transmission power must bemaintained to minimize interference problems.

Signals are transmitted as a succession of frames according to the IS95standard. As FIG. 4 of the accompanying drawings shows, each frame is ofduration 20 ms, and comprises sixteen 1.25 ms time slots. In each timeslot several bits of user data and/or control information can betransmitted.

Power control of transmissions from the MS 10 to the BTSs 4 (uplinkpower control) in IS95 is achieved as follows. When a BTS 4 receives asignal from the MS 10 it determines whether a predetermined property ofthe received signal (for example absolute signal level, signal to noiseratio (SNR), signal-to-interference ratio (SIR), bit error rate (BER) orframe error rate (FER)) exceeds a pre-selected threshold level. Based onthis determination, the BTS 4 instructs the MS 10 either to reduce or toincrease its transmission power in the next time slot.

For this purpose, two bits in every time slot of a pilot channel (PCH)from the BTS 4 to the MS 10 are allocated for uplink power control (seeFIG. 4). Both bits have the same value, and accordingly will be referredto hereinafter as the “power control bit” (or PCB) in the singular. Thepower control bit is assigned a value of zero by the BTS 4 if the MS 10is required to increase transmission power by 1 dB, and a value of oneif the MS 10 is required to decrease transmission power by 1 dB. The BTS4 is not able to request directly that the MS 10 maintain the sametransmission power; only by alternately transmitting ones and zeros inthe power control bit is the transmission power maintained at the samelevel.

When the MS 10 is in a SHO region 9, the MS 10 is required to make adecision on whether to increase or to decrease uplink transmission powerbased on a plurality of power control bits received respectively fromthe BTSs 4 involved in the soft hand-off. Consequently, an OR functionis performed on all the power control bits. If the result of this ORfunction is zero then the MS 10 will increase power on uplinktransmissions, and if the result is one then the MS 10 will decreasepower on uplink transmissions. In this way, uplink transmission power isonly increased if all BTSs 4 ask for an increase.

Power control of transmissions from the BTS 4 to the MS 10 (downlinkpower control) in IS95 is achieved as follows. When the MS 10 receives adownlink signal from a BTS 4 (or from each of a plurality of BTSs 4 insoft handoff operation) via a traffic channel (TCH), the FER of thatsignal is calculated by the MS 10 this reflects the degree to which thetraffic-channel signal has been corrupted by, for example, noise. ThisFER is then relayed by the MS 10 to the BTS 4 which transmitted thedownlink signal concerned, and the BTS 4 uses this FER to decide whetherto make any change to its downlink transmission power.

The soft hand-off system described above is effective in improvingsignal transmission between the MS 10 and the network when the MS 10 islocated in regions of cell overlap near the boundaries of the individualcells. Signal quality in these regions when using a single BTS 4 may berelatively poor, but by making use of more than one BTS 4 the qualitymay be substantially improved.

However, the IS95 soft hand-off system has the disadvantage ofincreasing signal traffic (“backhaul”) in the fixed network 5 since itis necessary to transmit signals carrying the same data and/or controlinformation between the BSC 6 and every BTS 4 involved in the softhand-off for both the uplink and downlink cases described above. Thisduplication of information is undesirable for two main reasons. Firstly,it leads to more traffic congestion in the fixed network. Secondly,higher costs are experienced by the mobile service provider (andconsequently the mobile service user), who may not own the fixed networkinfrastructure.

Therefore it is desirable to provide an improved soft hand-off methodcapable of affording the usual benefits of soft hand-off while at thesame time reducing the load on the fixed network.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided acellular mobile communications network including: a mobile station; aplurality of base transceiver stations (base stations), each forreceiving uplink signals from the mobile station; and a base stationcontroller connected to the base transceiver stations for receivingthere from such uplink signals; wherein the mobile station is operable,during a soft hand-off operation involving more than one of the basetransceiver stations of the network, to include, in one or more suchuplink signals transmitted thereby, respective signal measures for allof the base transceiver stations involved in the operation, each signalmeasure serving to indicate the performance of a communications channelbetween the mobile station and the base transceiver station concerned;and at least one of the base transceiver stations includes a processingunit operable, when that station is involved in such a soft hand-offoperation, to determine, based on an assessment of the signal measure(s)for one or more of the other base transceiver stations involved in thesoft hand-off operation, not to forward to the said base stationcontroller means such an uplink signal received from the mobile station.

According to a second aspect of the present invention there is provideda mobile station, for use in a cellular mobile communications network,including: a transmitter for transmitting uplink signals to a basetransceiver station of the network; and a selection processor connectedto the transmitter means and operable, during a soft hand-off operationinvolving a plurality of such base transceiver stations of the network,to cause the transmitter to include, in one or more of the uplinksignals, respective signal measures for all the base transceiverstations involved in the operation, each such signal measure serving toindicate the performance of a communications channel between the mobilestation and the base transceiver station concerned.

According to a third aspect of the present invention there is provided abase transceiver station, for use in a cellular mobile communicationsnetwork, including: a receiver for receiving uplink signals from amobile station of the network, one or more of which uplink signalsincludes, when the mobile station is engaged in a soft hand-offoperation involving the base transceiver station and at least onefurther base transceiver station of the network, respective signalmeasures for all the base transceiver stations involved in theoperation, each signal measure serving to indicate the performance of acommunications channel between the mobile station and the basetransceiver station concerned; and a soft hand-off controller operable,when the claimed base transceiver station is involved in such a softhand-off operation, to determine, based on an assessment of the signalmeasure(s) for one or more of the other base transceiver stationsinvolved in the operation, not to forward to the base station controllerof the network such an uplink signal received from the mobile station.

According to a fourth aspect of the present invention there is provideda soft hand-off control method for use in a cellular mobilecommunications network, wherein: when a soft hand-off operationinvolving more than one base transceiver station of the network is beingperformed, a mobile station of the network includes, in one or moreuplink signals transmitted thereby, respective signal measures for allthe base transceiver stations involved in the operation, each signalmeasure serving to indicate the performance of a communications channelbetween the mobile station and the base transceiver station concerned;and in at least one of the involved base transceiver stations, thesignal measure(s) of one or more of the other base transceiver stationsinvolved in the operation is/are assessed and a determination is made,based on the assessment, whether or not to forward to a base stationcontroller of the network an uplink signal received from the mobilestation.

According to a fifth aspect of the present invention there is provided acellular mobile communications network including: a mobile station; aplurality of base transceiver stations, each for transmitting downlinksignals to the mobile station and for receiving uplink signals from themobile station; and a base station controller connected to the basetransceiver stations for applying thereto such downlink signals; whereinthe mobile station is operable, during a soft hand-off operationinvolving more than one of the base transceiver stations of the network,to produce respective signal measures for all the base transceiverstations involved in the operation, each signal measure serving toindicate the performance of a communications channel between the mobilestation and the base transceiver station concerned; and the networkincluding a base transceiver station selector, operable to employ theproduced signal measures to determine which of the base transceiverstations involved in the operation should be used to transmit asubsequent one of the said downlink signals to the mobile station, andto cause that subsequent downlink signal to be transmitted by the basestation controller only to the determined base transceiver station(s).

According to a sixth aspect of the present invention there is provided amobile station, for use in a cellular mobile communications network,including: a transmitter for transmitting uplink signals to a basetransceiver station of the network; and a selection processing unitconnected to the transmitter and operable, during a soft hand-offoperation involving a plurality of such base transceiver stations of thenetwork, to produce respective signal measures for all the basetransceiver stations involved in the operation, each such signal measureserving to indicate the performance of a communications channel betweenthe mobile station and the base transceiver station concerned, and alsooperable to employ the produced signal measures to determine which ofthe involved base transceiver stations should be used to transmit asubsequent downlink signal to the mobile station, and to cause thetransmitter to include, in such an uplink signal transmitted thereby, abase transceiver station selection message identifying the determinedbase transceiver station(s).

According to a seventh aspect of the present invention there is provideda base station controller, for use in a cellular mobile communicationsnetwork to apply downlink signals to a plurality of base transceiverstations of the network, including: a receiver for receiving uplinksignals from one or more of the base transceiver stations, at least oneof which uplink signals includes, when a mobile station is engaged in asoft hand-off operation involving more than one of the base transceiverstations of the network, a base transceiver station selection messageidentifying which of the involved base transceiver stations should beused to transmit a subsequent one of the downlink signals to the mobilestation; and a soft hand-off controller operable to receive the uplinksignal including the base transceiver station selection message and totransmit the subsequent downlink signal only to the determined basetransceiver station(s) identified in the message.

According to an eighth aspect of the present invention there is provideda soft hand-off control method for use in a cellular mobilecommunications network, wherein: when a soft hand-off operationinvolving more than one base transceiver station of the network is beingperformed, a mobile station produces respective signal measures for allthe base transceiver stations involved in the operation, each suchsignal measure serving to indicate the performance of a communicationschannel between the mobile station and the base transceiver stationconcerned; and the produced signal measures are employed to determinewhich of the involved base transceiver stations should be used totransmit a subsequent downlink signal to the mobile station, and thesubsequent downlink signal is transmitted by a base station controllerof the network only to the determined base transceiver station(s).

The signal measures can be any suitable measure of thecommunications-channel performance between the mobile station and thebase transceiver stations, for example signal strength measures(received signal strength in terms of power or amplitude or qualitymeasures (frame error rate, signal-to-interference ratio, etc), or acombination of both strength and quality.

In preferred embodiments of the first to fourth aspects of the presentinvention the signal measures are respective power control bits receivedby the mobile station from the base transceiver stations involved in thesoft hand-off operation. These power control bits indicate whether ornot the mobile station is to increase or decrease its uplinktransmission power to the base transceiver station and therefore serveconveniently as measures of the uplink channel performance channelbetween the mobile station and each base transceiver station.

In a further embodiment a cellular mobile communications network where amobile station is capable of receiving a downlink signal from each of aplurality of base stations and transmitting an uplink signal to theplurality of base stations through a wireless channel, the networkcomprising: a selector unit for producing a measure of signal quality ofthe downlink signals from the plurality of base stations to the mobilestation and for selecting a base station from which the downlink signalshows a preferred signal quality; a transmitter for transmitting theuplink signal indicating the selected base station among the pluralityof base stations for subsequent communication with the mobile station;and a processing unit for processing the uplink signal to identify theselected base station, from among the plurality of base stations.

In this embodiment of the cellular mobile communications network, themobile station may include the selector and transmitter. The transmittermay be operable to include an identification of the selected basestation in the uplink signal. Further each base station includes theprocessing unit. The signal quality of the downlink signals from theplurality of base stations to the mobile station is represented bysignal strengths of the received downlink signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, discussed hereinbefore, shows parts of a cellular mobiletelecommunication network according to IS95;

FIG. 2, also discussed hereinbefore, shows a schematic view for use inexplaining processing of uplink signals in a soft hand-off operationperformed by the FIG. 1 network;

FIG. 3, also discussed hereinbefore, shows a schematic view for use inexplaining processing of downlink signals in such a soft hand-offoperation;

FIG. 4, also discussed hereinbefore, illustrates the format of a timeframe in the FIG. 1 network;

FIG. 5 shows parts of a mobile telecommunication network embodying thepresent invention;

FIG. 6 shows parts of a mobile station embodying to the presentinvention;

FIG. 7 is a flowchart for illustrating uplink processing operations inthe FIG. 6 mobile station;

FIG. 8 shows parts of a base transceiver station (base station)embodying the present invention;

FIG. 9 is a flowchart for illustrating uplink processing in the FIG. 8base transceiver station;

FIG. 10 shows an example decision table employed in the uplinkprocessing by the FIG. 8 base transceiver station;

FIG. 11 shows parts of another base transceiver embodying the presentinvention;

FIG. 12 shows a further example decision table employed in the uplinkprocessing by the FIG. 10 base transceiver station; and

FIG. 13 shows parts of a base station controller embodying the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 5 shows parts of a mobile telecommunication network embodying thepresent invention. In FIG. 5, elements that are the same as elements ofthe FIG. 1 network described previously have the same reference numeralsand an explanation thereof is omitted.

The FIG. 5 network is a wideband CDMA (W-CDMA) network for a proposednew standard for mobile telecommunications, referred to as a universalmobile telecommunications system (UMTS) or UMTS terrestrial radio access(UTRA). This is generally similar to the IS95-standard network describedpreviously, although certain implementation details are yet to befinalized. Details that are different from IS95 include the frameduration, which is 10 ms, and the time-slot duration which is 625 μs.The overall bit rate is within the range from 8 kbits/s to 2Mbits/s.Also downlink power control in W-CDMA is closed-loop and is based on thesame principles as the uplink power control.

In FIG. 5, each of three base transceiver stations (BTSs) 20 (BTS1, BTS2and BTS3) is connected via a fixed network 5 to a base stationcontroller (BSC) 30, which is in turn connected to a mobile switchingcenter (MSC) 7. Each BTS 20 serves a cell 8. A mobile station (MS) 40 isin a soft hand-off (SHO) region 9 and can receive downlink signals from,and transmit uplink signals to, all the BTSs 20 involved in the softhand-off.

The FIG. 5 network corresponds generally with the FIG. 1 network, butthe MS 40, BTSs 20 and BSC 30 are constructed and operate differentlyfrom the corresponding elements in FIG. 1.

FIG. 6 is a block diagram showing parts of a MS 40 embodying the presentinvention. An antenna element 42 is connected (e.g. via a duplexer—notshown) to a receiver portion 44 and a transmitter portion 46. A signalselection information processing portion 48 receives from the receiverportion 44 respective downlink signals DS1 to DS3 produced by the threeBTSs BTS1 to BTS3 involved in the soft hand-off operation. The signalselection information processing portion 48 applies a ranking message RMand a power control message PCM to the transmitter portion 46.

FIG. 7 is a flow chart showing the actions performed by the signalselection processing portion 48 of the MS 40 when performing uplinkprocessing while the MS is in the soft hand-off region 9. Firstly, instep A1, the three BTSs 20 are ranked based on a predetermined propertyof the respective downlink signals DS1 to DS3 that are being received bythe MS 40, for example received signal strength (RSS). Alternatively,the ranking may be based on a “first-come first-served” basis, i.e. onthe order in which BTSs 20 became involved in the soft hand-offoperation. Alternatively, the ranking could be random. In step A2 aranking message RM, indicating the order in which the BTSs are presentlyranked, is then sent via a control channel to all BTSs 20. After theranking message is sent, processing continues to step A3.

The loop from steps A3 to A6 occurs once for every time slot of thetraffic channel (TCH) and its associated control channel (DCCH) in thedownlink direction. As was the case for the IS95 uplink power controlmethod described above, every time slot of the TCH/DCCH from BTS 20 toMS 40 contains a power control bit for the purpose of instructing the MS40 to increase or reduce its uplink transmission power. In step A3, sucha power control bit is received from each of the three BTSs 40 involvedin the soft hand-off.

In step A4, the plurality of power control bits received in step A3 arearranged into a power control message (PCM) in rank order according tothe current BTS ranking decided in step A1. Following this, in step AS,the PCM is transmitted to all involved BTSs via a control channel.

The ranking decided in step A1 may periodically require updating, forseveral reasons. Firstly, as the MS 40 moves, a downlink signal may bereceived from a new BTS or an existing BTS may no longer be able toprovide a detectable downlink signal. Secondly, the qualities of thesignals received from the BTSs 20 may have changed, e.g. due to fading.Therefore, in step A6 it is decided whether or not a ranking update isrequired. Such an update may be carried out periodically at regular timeintervals (for example every several hundred milliseconds as in GSMnetworks), or every frame or even every time slot. Alternatively, theranking could be updated only when a new BTS is detected or contact withan existing one lost. If an update is required, processing is returnedto step A1, otherwise processing returns to step A3 for the start of thenext time slot.

FIG. 8 is a block diagram showing parts of a BTS 20 embodying thepresent invention. This BTS 20 is specially adapted to receive andprocess the ranking message RM sent by the MS 40 in step A2 of FIG. 7and the power control message PCM sent by the MS 40 in step A5.

An antenna element 22 is connected (e.g. via a duplexer—not shown) to areceiver portion 24 and a transmitter portion 26. A soft hand-offcontrol portion 28 receives an uplink signal US from the receiverportion 24, and in turn applies the received US (or a signal derivedtherefrom) to the fixed network 5 for transmission to the BSC 30.Optionally contained within the soft hand-off control portion 28 is astorage portion 29.

In use of the BTS 20, the uplink signals sent by the MS 40 when it is inthe soft hand-off region 9 include, from time to time, a ranking messageRM. The uplink signals US detected by the receiver portion 24 in the BTS20 are applied to the soft hand-off control portion 28. When the softhand-off control portion 28 detects that a ranking message RM isincluded in one of the uplink signals US received thereby, it processesthe ranking message concerned to determine the rank of its BTS withinthe ranking order determined by the MS in step A1 described above.

In each time slot, the uplink signals US produced by the receiverportion 24 also include a power control message PCM determined by the MS40 as described above in step A4 of FIG. 7.

Operation of the soft hand-off control portion 28 in response to thepresence of such a PCM in the uplink signal US produced by the receiverportion 24 will now be described with reference to FIG. 9.

It is assumed that, by the time the sequence shown in FIG. 9 iscommenced, a ranking message RM has already been received and processed(as indicated above) by the soft hand-off control portion 28.

In FIG. 9, in step B1 the PCM is received by the soft hand-off controlportion and examined.

In step B2, the soft hand-off control portion 28 determines whether itsBTS 20 specified, in its last power control bit (PCB) sent to thesubject MS 40, that the MS 40 should reduce its uplink transmissionpower (PCB=1). If so, processing proceeds to step B3.

In step B3, the soft hand-off control portion 28 goes on to examine thePCM which includes the respective last PCBs of all of the other BTSsinvolved in the present soft hand-off operation. If any of those PCBs is1, this denotes that at least one other BTS requested the subject MS 40to reduce its uplink transmission power. In this case, processingproceeds to step B4.

In step B4, the soft hand-off control portion 28 determines whether ornot, in the order of ranking presently determined by the MS 40, its BTSis ranked higher than each other BTS that requested the MS to reduce itsuplink transmission power.

If its BTS is the highest-ranked BTS that has requested a powerreduction, processing proceeds to step B5 in which the soft hand-offcontrol portion 28 determines that its BTS is required to send theuplink signal US received in the current time slot to the BSC 30 via thefixed network 5.

If in step B4 the soft hand-off control portion 28 determined thatanother BTS, having a higher rank than its BTS, also asked for a powerreduction, processing proceeds to step B7 in which the soft hand-offcontrol portion 28 determines that it is not required to transmit theuplink signal US received from the mobile station 40 in the current timeslot to the BSC 30.

In step B3, if the soft hand-off control portion 28 determines that itsBTS was the only BTS involved in the soft hand-off operation to ask fora power reduction, processing proceeds to step B5 in which the uplinksignal US for the current time slot is transmitted by the BTS to the BSC30.

If in step B2 the soft hand-off control portion 28 determines that itasked the MS 40 for a power increase (i.e. its last PCB was 0),processing proceeds to step B6. In step B6 the soft hand-off controlportion 28 determines, by referring to the PCM, whether any other BTSasked for a reduction (i.e. the last PCB specified by that other BTS was1). If so, the soft hand-off control portion 28 determines that its BTSis not required to transmit the uplink signal US to the BSC 30 in thecurrent time slot and processing proceeds to B7. If, on the other hand,no other BTS requested a power reduction (i.e. all BTSs involved in thepresent soft hand-off operation requested an increase in the MS uplinktransmission power), processing proceeds to step B5 and the US for thecurrent time slot is transmitted by the BTS to the BSC 30.

After step B5 or B7 (as the case may be) processing for the current timeslot is completed and the soft hand-off control portion 28 awaits thenext PCM or RM from the MS 40.

As described above with reference to FIG. 9, by virtue of its receipt ofthe PCM, the soft hand-off control portion 28 in each BTS involved in asoft hand-off operation has knowledge of the last power control bit sentto the subject MS 40 by all of the other BTSs, as well as by its ownBTS. By comparing these PCBs, the soft hand-off control portion in eachBTS can decide whether or not to transfer the uplink signal US receivedin the current time slot to the BSC, such that, whenever possible, onlyone of the BTSs involved in the soft hand-off transfers the uplinksignal US to the BSC.

Based on the received PCBs, the soft hand-off control portion 28 in each“deciding BTS” identifies whether the power reduction/increase requestsby the different BTSs fall into one of four different cases.

Case 1: If the deciding BTS has asked for a power increase while atleast one other BTS has asked for a power reduction, it suggests that atleast one other BTS is enjoying a better-quality uplink signal from theMS 40. Accordingly, this other BTS, rather than the deciding BTS, shouldsend the uplink signal US in the current time slot to the BSC. Thedeciding BTS therefore decides not to send the uplink signal US.

Case 2: If the deciding BTS has requested a power reduction but everyother BTS involved in the soft hand-off operation has requested anincrease in power, the deciding BTS determines that it is receiving thebest-quality signal from the MS and decides to transmit the US in thecurrent time slot to the BSC.

Case 3: If the deciding BTS has asked the MS to reduce power, and atleast one other BTS has also asked for such a power reduction, thedecision as to which BTS is to transfer the US is based on rank. Forexample, the highest-ranked of the BTSs requesting a power reductiondetermines that it should transfer the US in the current time slot tothe BSC. Thus, case 3 is divided into two sub-cases 3a and 3b. In case3a the deciding BTS determines that a higher-ranked BTS has asked for apower reduction and so determines that it should not send the US. Incase 3b, on the other hand, the deciding BTS determines that it is thehighest-ranked of the BTSs requesting a power reduction, and transfersthe US to the BSC 30.

Case 4: If all the BTSs involved in the soft hand-off operation haverequested the MS to increase its transmission power, all of the BTSstransfer their respective uplink signals US in the current time slot tothe BSC, as in the conventional soft hand-off operation describedpreviously with reference to FIG. 2. This permits maximum ratiocombining (MRC) processing of the different uplink signals at the BSC30.

As described above, the ability to make decisions at the BTS, ratherthan at the BSC, facilitates a significant reduction in thefixed-network backhaul for uplink processing in the soft hand-offoperation.

FIG. 10 shows an example decision table for use in illustratingoperation of the soft hand-off control portion 28 during uplinkprocessing. In this example, it is assumed that the BTSs involved in thesoft hand-off operation are ranked as follows: BTS3 has rank {circlearound (1)} (the highest rank); BTS1 has rank {circle around (2)}; andBTS2 has rank {circle around (3)} (the lowest rank).

As illustrated in FIG. 10, the MS 40 arranges the power control bitsPCBs for the different BTSs in the power control message PCM in theorder of rank of the BTSs. Thus, the first bit in the PCM corresponds tothe rank-{circle around (1)} BTS (BTS3 in this example); the second bitin the PCM corresponds to the rank-{circle around (2)} BTS (BTS1); andthe third bit in the PCM corresponds to the rank-{circle around (3)} BTS(BTS2).

In this example, it is also assumed that the deciding BTS is BTS1 (whichin this case is the middle-rank BTS).

In case 1 above, the PCM=001, indicating that BTS2 alone has requested apower reduction. Thus, BTS2 should transmit the uplink signal for thecurrent time slot and BTS1 determines that it should not transmit theuplink signal.

In case 2, the PCM=010, indicating that the deciding BTS1 alone hasrequested a power reduction. Accordingly, BTS1 determines that it shouldtransmit the uplink signal US to the BSC.

In case 3a, both BTS3 and BTS1 have requested a power reduction, whereasBTS2 has requested a power increase. In this case, the deciding BTS1refers to its rank in the order of ranking determined by the MS andestablishes that, as the first PCM bit (corresponding to the higher-rankBTS3) is 1, it (the deciding BTS1) should not transmit the US to theBSC.

In case 3b, on the other hand, the PCM=011, indicating that both BTS1and BTS2 have requested a power reduction and BTS3 has requested a powerincrease. In this situation, the deciding BTS1 determines that no BTS ofrank higher than it has requested a power reduction (the first PCM bitis 0) and therefore decides to transmit the US to the BSC.

Finally, in case 4, the PCM=000 which indicates that all BTSs haverequested a power increase. In this case, the deciding BTS1 determinesthat it should transmit the US to the BSC.

It will be appreciated that it is not essential for the decision-makingcarried out by the BTSs involved in the soft hand-off operation toresult in only one of the BTSs transmitting the US to the BSC in thecurrent time slot in the cases 1, 2, 3a and 3b. For example, somebenefit would still be achieved, in terms of reducing the fixed-networkbackhaul for uplink processing, as long as at least one BTS decides notto transmit the US in any of the cases 1, 2, 3a or 3b.

It will also be appreciated that, in order to avoid erroneous decisionmaking in the BTSs, based for example on temporary phenomena in theuplink signal reception at the BTSs, it may be preferable for the BTSsto make their uplink-signal transmission decisions based on a history ofthe power control bits sent to the MS. For example, the storage portion29 included within each soft hand-off control portion 28 could be usedto store one or more previous PCMs received by the BTS. Using this PCMhistory, as stored in the storage portion 29, each BTS could make a moreinformed decision as to whether or not to transmit the uplink signal tothe BSC.

For example, if the history of the PCMs shows that each BTS is sendingalternate ones and zeros to the MS (indicating generally that the signalconditions between the MS and each BTS involved in the soft hand-offoperations are effectively static), it would be unproductive for thetransmitting BTS to continuously “swap around” as a result of thealternating ones and zeros. Such swapping around could be eliminated,for example, by providing each soft hand-off control portion 28 with afacility to identify a “don't care” reception situation (such as astream of alternating ones and zeros) for each BTS involved in the softhand-off operation. In this “don't care” situation, the soft hand-offcontrol portion 28 could simply decide to apply the last decision itmade as to whether or not to transmit the uplink signal to the BSC thistime around, so eliminating the swapping around phenomenon. Other “don'tcare” situations could also be identified, for example by applying amoving average to the sequence of PCBs received for any given BTS.

Similarly, a moving average could be used to make the decision as towhether the reception conditions fall into any of cases 1 to 4 in FIG.10. In this case, instead of “1” or “0” in FIG. 10 representing just thePCB in the current PCM, “1” or “0” could represent the moving average(rounded up or down to 1 or 0) for the BTS concerned over the past (say)4 PCMs.

It will also be understood that it is not necessary for the uplinkprocessing to take place every time slot. It would also be possible forthe PCM to be transmitted only once per frame, in which case thedecision-making applied by each BTS would be made on a frame-by-framebasis.

Furthermore, it would even be possible for the decisions to be made attime intervals other than frames or time slots, for example based on atime interval consistent with the fading characteristics of the RFchannels in the network.

In the embodiment described above, when two or more BTSs involved in thesoft hand-off operation have comparably-good uplink channelperformances, the BTS used to transmit the uplink signal to the BSC isselected based on the BTS ranking determined by the mobile stationalone. However, it is not essential for the ranking of the BTSs to beperformed exclusively by the MS and it is possible for the ranking (orpart of it) to be performed elsewhere in the network (e.g. in the BSC)based on other criteria.

For example, in a preferred embodiment the BTSs may be ranked accordingto a first ranking determined by the mobile station as describedpreviously. This first ranking may be termed a ranking based on the airinterface between the mobile station and the BTSs. The BTSs may also beranked according to a second ranking determined by the BSC. This secondranking may be based on so-called “backhaul preference”, i.e. an orderof preference in which the BTSs should transfer (backhaul) the receiveduplink signal to the BSC. Factors which influence the backhaulpreference include: congestion and availability of the fixed-networkcommunication paths linking the different BTSs to the BSC; the qualityof those communication paths; and the cost of using those communicationspaths. In particular, the fixed network employed to provide thecommunications paths between the BTSs and the BSC is subject tocongestion so that availability problems may arise. Also, somecommunications paths such as microwave links may offer relatively lowquality compared to other types of communication path such asfiber-optic paths. Cost considerations also arise because thefixed-network operator may levy different charges for the use of thedifferent communications paths, including different charges fordifferent bandwidths and different tariffs at different times of use.

Accordingly, by ranking the BTSs in accordance with backhaul preference(as well as in accordance with air-interface performance), it ispossible to employ a combination of the backhaul preference determinedby the second ranking and the air-interface preference determined by thefirst ranking in suitable cases.

FIG. 11 shows parts of a BTS 120 for use in the above-described example.The FIG. 11 BTS 120 is constituted in basically the same way as the BTS20 of FIG. 8, but includes a modified soft hand-off control portion 128which receives a first ranking message RM1 from the mobile station and asecond ranking message RM2 from the BSC via the fixed-network connectionpath 5.

To this end the BSC in this embodiment further includes a communicationspath ranking portion (not shown in the drawings) which determines thebackhaul preference based on one or more of the factors mentioned aboveand transmits the second ranking message specifying the determinedbackhaul preference to the BTSs involved in the soft hand-off operation.

The soft hand-off control portion 128 employs a super decision-matrixwhen deciding whether or not its BTS 120 should forward an uplink signalUS received from the mobile station to the BSC.

FIG. 12 shows one example of the application of this super decisionmatrix.

In this example, it is assumed that four BTSs are involved in the softhand-off operation. In accordance with the first ranking message RM1provided to the soft hand-off control portion 128 by the mobile station,the four BTSs are ranked as follows: BTS1—rank {circle around (1)};BTS2—rank {circle around (3)}; BTS3—rank {circle around (2)}; BTS4—rank{circle around (4)}. According to the second ranking message RM2provided to the soft hand-off control portion 128 by the BSC, the BTSsare ranked differently as follows: BTS1—rank {circle around (4)};BTS2—rank {circle around (2)}; BTS3—rank {circle around (3)}; andBTS4—rank {circle around (1)}.

In this example it is also assumed that the power control bits (arrangedin a power control message PCM received from the mobile station) are (inorder from BTS1to BTS4) 0, 1, 1, 0. This signifies that BTS2 and BTS3are both enjoying comparably-good communications-channel performances.In this case (which corresponds to cases 3a and 3b in FIG. 10) adecision, as to which of these two candidate BTSs BTS2 and BTS3 shouldtransmit the received uplink signal in the next time slot to the BSC, ismade based on a combination of the two rankings (air-interface rankingprovided by the first ranking message RM1 and backhaul ranking providedby the second ranking message RM2).

In each of the BTSs concerned (BTS2 and BTS3), the soft hand-off controlportion 128 determines that it should follow the backhaul rankingpreference, which indicates that BTS2 rather than BTS3 should be used totransmit the uplink signal to the BSC, even though according to theair-interface ranking, BTS2 is inferior to BTS3. Such a decision ispossible because, in this case, the difference in air-interface rankingbetween the two candidate BTSs BTS2 and BTS3 is only one, indicatingthat BTS2 is only slightly inferior to BTS3. (It might not be desirableto follow the backhaul ranking preference had the two candidate BTSs hadbeen BTSs having very different air-interface rankings, for example BTS1and BTS4).

Thus, as described above, the decision-making in the soft hand-offcontrol portions of the different BTSs involved in the soft hand-offoperation can be based on one of the two rankings (air-interface rankingand backhaul ranking) alone or on a combination of both types ofranking. In particular, it will be understood that when the rankingapplied by the mobile station (air-interface ranking) is purely randomor based on the order of involvement of the BTSs in the soft hand-offoperation, it may well be preferable for the air-interface ranking to beoverridden completely by the backhaul ranking.

FIG. 6 is a block diagram showing parts of a MS 40 embodying the presentinvention. An antenna element 42 is connected (e.g. via a duplexer—notshown) to a receiver portion 44 and a transmitter portion 46. A signalselection information processing portion 48 from the receiver portion 44respective downlink signals DS1 to DS3 produced by the three BTSs BTS1to BTS3 involved in the soft hand-off operation. The signal selectioninformation processing portion 48 applies a ranking message RM and apower control message PCM to the transmitter portion 46.

Referring again to FIG. 6, to deal with the downlink processing, thesignal selection information processing portion 48 is required toperform a further function in addition to the generation of the rankingmessage RM and power control message PCM as described previously. Inthis case, as in the previously-described ranking process the signalselection information processing portion 48 again processes therespective downlink signals DS1 to DS3 received from the BTSs (BTS1 toBTS3) involved in the soft hand-off operation, and compares thesedownlink signals according to a predetermined property (which may be thesame property as for the uplink processing case or another property, asdesired). In a preferred embodiment, the predetermined property is thereceived signal strength (RSS), possibly together with thesignal-to-interference ratio (SIR). These performance measures aredetermined for the downlink DCCH.

The signal selection information processing portion 48 employs theperformance measures to select which of the BTSs involved in the softhand-off operation is to be used to transmit the downlink signal to theMS in the next time slot.

The signal selection information processing portion 48 may select theBTS that is to transmit the downlink signal in the next time slot basedon, for example, the following cases.

Case 1: If the RSS (and/or SIR) of a single BTS is higher than eachother BTS, that single BTS is selected to transmit the downlink signalin the next time slot.

Case 2: If two or more BTSs have comparably-good RSS (and/or SIR), oneof them is selected based on an order of ranking (e.g. order ofinvolvement in the soft hand-off operation or random).

Case 3: If all the BTSs involved in the soft hand-off operation fail tomeet a prescribed RSS (and/or SIR) threshold, all the BTSs are selectedto transmit the downlink signal in the next time slot, so that a MRCoperation can be performed at the MS 40 to give the best chance ofobtaining a useful signal.

After determining which BTS(s) is/are to be used, the signal selectioninformation processing portion 48 transmits a BTS selection message(BSM), identifying the BTS(s) to be used, to all of the BTSs on acontrol channel.

For example, using two bits to provide the BSM, the BSM may be set to“01” to designate BTS1; “10” to designate BTS2; and “11” to designateBTS3. “00” denotes that all the BTSs should be used to transmit thedownlink signal in the next time slot.

Each BTS receives the BSM via the control channel from the MS 40. One ormore of the BTSs then forward the BSM to the BSC 30. As describedpreviously with reference to FIGS. 8 to 10, only one BTS may decide totransmit the uplink signal including the BSM to the BSC, by applying thedecision-making strategy described previously for the uplink processing.However, the number of BTSs that forward the BSM to the BSC isirrelevant to this aspect of the invention, and all BTSs could forwardthe BSM to the BSC.

FIG. 13 shows part of a BSC adapted to perform downlink processing inthe soft hand-off operation. The BSC 30 includes a control portion 32and a selector portion 34.

In this example, it is assumed that the connection lines 5 ₁ to 5 ₃linking each BTS to the BSC 30 are duplex lines, which carry respectiveuplink and downlink signals US, and DS between the BTS concerned and theBSC. For example, a first connection line 5 ₁ carries respective uplinkand downlink signals US1 and DS1 between the BTS1 and the BSC 30.

The selector portion 34 receives at its input a downlink signal DSsupplied by the MSC (7 in FIG. 5). The selector portion 34 has threeoutputs connected respectively to the connection lines 5 ₁ to 5 ₃.

The selector portion 34 also has a control input which receives aselection signal SEL. In response to the SEL selection signal theselector portion 34 connects its input to one, or all, of its threeoutputs.

The control portion 32 also has three inputs connected respectively tothe connection lines 5 ₁ to 5 ₃ for receiving the uplink signals US1 toUS3 from BTS1 to BTS3 respectively. The control portion applies theselection signal SEL to the selector portion 34.

In operation of the BSC shown in FIG. 13, in each time slot of theuplink signal the control portion 32 receives one or more of the threeuplink signals US1 to US3 from the BTSs involved in the soft hand-offoperation. When the BSM supplied by the MS 40 is detected within areceived uplink signal US1, US2 or US3, the control portion 32 examinesthe BSM and determines therefrom which of the BTSs is to be used totransmit the downlink signal in the next time slot to the MS 40.

If the BSM designates a single BTS, the control portion 32 sets theselection signal SEL such that the selector portion 34 supplies thedownlink signal DS just to that one of the connection lines 5 ₁ to 5 ₃connecting the BSC 30 to the designated BTS. If, on the other hand, allBTSs are designated by the BSM, the selection signal SEL is set so thatthe downlink signal DS received from the MSC 7 is supplied to all of theconnection lines 5 ₁ to 5 ₃.

It will be appreciated that it is not necessary for the downlinkprocessing to be performed on a time slot-by-time slot basis. It couldbe performed on a frame-by-frame basis or the BTS selection could bemade at some other suitable time interval.

It would also be possible for the signal selection informationprocessing portion 48 (FIG. 6) to include its own storage portion(similar to the storage portion 29 in FIG. 8) enabling it to store apast history of the RSS (and/or SIR) measures for the different BTSscurrently involved in the soft hand-off operation. In this case, asdescribed previously in relation to the uplink processing, it would bepossible for the MS to employ more sophisticated decision-making inrelation to the BTS selection so as to avoid undesirable effects causedby temporary reception phenomena or other problems caused by toofrequent-changing of the BTS selection.

It is not necessary for the mobile station to carry out the comparisonof the signal measures for the different downlink signals and make thedetermination of the BTS to be used to transmit the downlink signal. Thecomparison and BTS determination could be carried out in the BSC; inthis case instead of transmitting the BSM to the BTSs involved in thesoft hand-off operation, the mobile station could transmit the downlinksignal measures themselves (in some suitable form). These measures wouldthen be delivered in the usual way to the BSC, enabling it to comparethem and then make the BTS determination.

Although the present invention has been described above in relation tothe proposed European wideband CDMA system (UTRA) it will be appreciatedthat it can also be applied to a system otherwise in accordance with theIS95 standard. It would also be possible to apply the invention in othercellular networks not using CDMA, for example networks using one or moreof the following: multiple-access techniques: time-division multipleaccess (TDMA), wavelength-division multiple access (WDMA),frequency-division multiple access (FDMA) and space-division multipleaccess (SDMA).

I claim:
 1. A base station which supports a wireless channel for amobile station together with one or more other base stations in a softhandoff mode, the base station transmitting and receiving, with themobile station, radio frames having a predetermined length, comprising:a transmitter which transmits a downlink signal to the mobile station,the mobile station being capable of receiving the downlink signal anddownlink signals from the other base stations in the soft handoff mode;a receiver that receives an uplink signal from the mobile station; and acontroller that identifies if the base station is a selected basestation upon the base station receiving, from the mobile station, anuplink signal indicating the selected base station for subsequentcommunication with the mobile station from which the downlink signalshows a preferred signal quality; wherein when said base station is theselected base station, said base station transmits the next downlinksignal, otherwise said base station maintains the soft handoff mode withthe mobile station without sending the next downlink signal.
 2. The basestation as claimed in claim 1, wherein the uplink signal includes anidentification of the selected base station.
 3. The base station asclaimed in claim 1, wherein the signal quality is represented by signalstrengths of the downlink signals.
 4. The base station as claimed inclaim 1, wherein said controller further controls downlink power of saidbase station according to whether said base station is a selected basestation.
 5. The base station as claimed in claim 4, wherein saidcontroller further identifies power control information included in theuplink signal and said controller controls downlink power according tothe power control information and whether said base station is aselected base station.
 6. The base station as claimed in claim 4,wherein the one or more base stations in soft handoff mode with themobile station that are not selected base stations maintain the downlinkchannel even when not transmitting the downlink signal in the softhandoff mode.
 7. The base station as claimed in claim 1, wherein saidbase station periodically receives an uplink signal indicating theselected base station and said selected base station transmits the nextdownlink frame.
 8. The base station as claimed in claim 7, wherein saidframe includes a plurality of time slots and said selected base stationtransmits a next downlink time slot.
 9. A base station which supports acommunication channel with a mobile station where the mobile station mayestablish and maintain another communication channel with one or moreother base stations in a soft handoff mode such that the mobile stationmay simultaneously communicate with a plurality of base stations,comprising: a transmitter which transmits a downlink communicationsignal to the mobile station, the mobile station being capable ofreceiving, in the soft handoff mode, the downlink communication signaland downlink communication signals from the other base stations; areceiver that receives an uplink communication signal from the mobilestation; and a controller that identifies if the base station is aselected base station upon the base station receiving, from the mobilestation, an uplink signal indicating the selected base station fortransmitting a next downlink communication signal, the selected basestation from among the plurality of base stations in the soft handoffmode with the mobile station, the mobile station selecting one or morebase stations by identifying among downlink signals having apredetermined property; wherein when said base station is the selectedbase station, said base station transmits the next downlink signal,otherwise said base station maintains the communication channel softhandoff mode with the mobile station without sending the next downlinkcommunication signal.
 10. The base station as claimed in claim 9,wherein in the soft handoff mode the mobile station is capable ofreceiving the downlink communication signals containing identicalcommunication data transmitted nearly simultaneously from the pluralityof base stations in soft handoff communication with the mobile station,and the mobile station identifies among downlink signals from theplurality of base stations, in soft handoff communication, down linksignals showing a preferred signal quality as the predetermined propertyand transmits signal selection information with an identification of theselected base station to the plurality of base stations, in soft handoffcommunication.